Inbreeding in open pollinated corn

​Separation of tassel and ear in the corn plant prevented severe selfing but individual farmers in the late 1800’s probably did not make large enough saves to avoid their versions of a widely-used variety to become too genetically narrow. It is probable that ‘negative’ recessive genes would become homozygous whereas new varieties developed by crossing unrelated varieties would show more yield.  It did become apparent by 1900 that crossing distinct genetic populations resulted in a vigor boost.
 
The strength of maintaining some genetic variability with open pollinated seed also included the weakness of the population genetically narrowing and at the same time genetically drifting with annual environmental pressures. Avoidance of the drift could only be done by selfing plants to make them homozygous and therefore more repeatable.  USDA researchers in the early 1900’s started developing inbreds from the various populations and discovering hybrid vigor.  Those early inbreds, however, were deemed as producing too little seed to be economic.  The solution was to cross related inbreds to make hybrids as parents.  These double crosses still showed advantages over open pollinated varieties and became popular in USA from 1930-1950.  These gave way to three-way crosses in which only the female parent was a hybrid. These and double crosses had less variability than open pollinated varieties but still varied in individual plant characters such as flowering dates and plant heights.  Not only did the move to more mechanized corn production favor uniformity but also, as plant densities increased, completion for light favored plants of the same height.  Studies done at University of Missouri in the 70’s reinforced the view of many corn specialists that short plants among taller ones yield less than when the short plants are planted separately. 
 
As inbred yields increased and seed production practices improved, the move to single cross hybrids became fixed in the US corn belt in the 70’s.